Hereinafter trademarks or trade names are shown in upper case characters.
Perfluoropolyethers (hereinafter PFPE) are fluids having important uses in oils and greases for use under extreme conditions. A property shared by the class is extreme temperature stability in the presence of oxygen and they find use in tribological or lubrication applications. Among their advantages as extreme lubricants is the absence of gums and tars among the thermal decomposition products. In contrast to the gum and tar thermal degradation products of hydrocarbons, the degradation products of PFPE fluids are volatile. In actual use, the upper temperature limit is determined by the stability of the oil or grease. Lewis acids, metal fluorides such as aluminum trifluoride or iron trifluoride, are formed as a result of heat at microscale loci of metal to metal friction; for instance as stationary bearings are started in motion. Thus the PFPE stability in the presence of the metal fluoride, although lower than the stability in the absence of the metal fluoride, establishes the upper performance temperature. The three commercial PFPEs, KRYTOX (from E.I. du Pont de Nemours and Company, Inc., Wilmington Del.), FOMBLIN and GALDEN (from Ausimont/Montedison, Milan, Italy) and DEMNUM (from Daikin Industries, Osaka, Japan) differ in chemical structure. A review of KRYTOX is found in Synthetic Lubricants and High-Performance Fluids, Rudnick and Shubkin, Eds., Marcel Dekker, New York, N.Y., 1999 (Chapter 8, pp. 215-237). A review of FOMBLIN and GALDEN is found in Organofluorine Chemistry, Banks et al., Eds., Plenum, New York, N.Y., 1994, Chapter 20, pp. 431-461, and for DEMNUM, in Organofluorine Chemistry (op. cit.), Chapter 21, pp. 463-467.
The anionic polymerization of hexafluoropropylene epoxide as described by Moore in U.S. Pat. No. 3,332,826 can be used to produce the KRYTOX fluids. The resulting poly(hexafluoropropylene epoxide) PFPE fluids are hereinafter described as poly(HFPO) fluids. The initial polymer has a terminal acid fluoride, which is hydrolyzed to the acid followed by fluorination. The structure of a poly(HFPO) fluid is shown by Formula 1:CF3—(CF2)2—O—[CF(CF3)—CF2—O]S—Rf  (Formula 1)where s is 2-100 and Rf is a mixture of CF2CF3 and CF(CF3)2, with the ratio of ethyl to isopropyl terminal group ranging between 20:1 to 50:1.
DEMNUM fluids are produced by sequential oligomerization and fluorination of 2,2,3,3-tetrafluorooxetane (tetrafluorooxetane), yielding the structure of Formula 2.F—[(CF2)3—O]t—Rf2   (Formula 2)where Rf2 is a mixture of CF3 or C2F5 and t is 2-200.
A common characteristic of the PFPE fluids is the presence of perfluoroalkyl terminal groups.
The mechanism of thermal degradation in the presence of a Lewis acid such as aluminum trifluoride has been studied. Kasai (Macromolecules, Vol. 25, 6791-6799, 1992) discloses an intramolecular disproportionation mechanism for the decomposition of PFPE containing —O—CF2—O— linkages in the presence of Lewis acids.
FOMBLIN and GALDEN fluids are produced by perfluoroolefin photooxidation. The initial product contains peroxide linkages and reactive terminal groups such as fluoroformate and acid fluoride. These linkages and end groups are removed by ultraviolet photolysis and terminal group fluorination, to yield the neutral PFPE compositions FOMBLIN Y and FOMBLIN Z represented by Formulae 3 and 4, respectivelyCF3O(CF2CF(CF3)—O—)m(CF2—O—)n—Rf3   (Formula 3)where Rf3 is a mixture of —CF3, —C2F5, and —C3F7; (m+n) is 8-45; and m/n is 20-1000; andCF3O(CF2CF2—O—)p(CF2—O)qCF3   (Formula 4)where (p+q) is 40-180 and p/q is 0.5-2. It is readily seen that Formulae 3 and 4 both contain the destabilizing —O—CF2—O— linkage since neither n nor q can be zero. With this —O—CF2—O— linkage in the chain, degradation within the chain can occur, resulting in chain fragmentation.
For PFPE molecules with repeating pendant —CF3 groups, Kasai discloses the pendant group provides a stabilizing effect on the chain itself and for the alkoxy end groups adjacent to a —CF(CF3)—. Absent the —O—CF2—O— linkage, the PFPE is more thermally stable, but its eventual decomposition was postulated to occur at end away from the stabilizing —CF(CF3)— group, effectively unzipping the polymer chain one ether unit at a time.
Therefore, there is substantial interest and need in increasing the thermal stability of PFPE fluids.